This invention relates to polyolefin adhesive compositions and articles made therefrom, and more particularly to polyolefin adhesive compositions having a fast set time.
A hot melt adhesive (HMA) is a thermoplastic polymer system applied in a molten state. The molten adhesives are applied to a substrate and then placed in contact with another substrate. The adhesive cools and solidifies to form a bond between the substrates. Hot melt adhesives are widely used in the packaging industry, for example, to seal and close cartons or to laminate multilayer papers.
The HMA should have good adhesion over a wide temperature range, e.g. at low temperatures for packaging applications in the frozen-goods sector, and a low viscosity melt to facilitate application to the substrate, especially for automatic processing. Where applied from a heated reservoir in which it is maintained as a melt, for example, typically at 177° C. (350° F.) in the prior art, for an extended period of time, the adhesive should have oxidative resistance and good thermal stability, which is also called “pot life.” The HMA should have a moderate to long open time, defined as the time span between adhesive application to a first substrate and assembly of the parts to be joined. On the other hand, a fast set time is required to quickly build up bond strength on fast-running packaging machines. The characteristic set time is defined as the time needed for the hot melt adhesive to solidify to the point where it possesses enough bond strength to form bonds to give substrate fiber tear when pulled apart, e.g., the bond is sufficiently strong such that sealed substrates will not pop open upon exit from the compression section on a packaging line. The bond may continue to build additional strength upon further cooling; however, compression is no longer required to maintain adhesion between the substrate surfaces. In reference to automatic packaging lines, the set time is referred to as the duration of the application of adhering pressure, which is generally at least equal to or greater than the characteristic set time of the HMA.
A wide variety of thermoplastic polymers, particularly ethylene-based polymers such as ethylene-vinyl acetate copolymers (EVA) have traditionally been used in HMA, but often the adhesive formulation requires substantial use of low molecular weight ingredients such as wax and tackifier to adjust the viscosity and glass transition temperature to useful ranges. Thus, EVA formulations are typically exemplified by a wax with either a low molecular weight or a low crystallization temperature. An HMA for packaging applications such as case and carton sealing is typically composed of a polymer, a tackifier or diluent, and a wax. The polymer has largely influenced the flow and mechanical properties: (a) viscosity and rheology characteristics, (b) cohesive strength, (c) flexibility, and (d) adhesive strength. The viscosity of the polymer has typically been a few orders of magnitude higher than the viscosity of the wax. Low viscosity wax has been used to reduce the high viscosity of the polymer and resin to ensure efficient mixing. This viscosity reduction is particularly important during the application stage where a low viscosity HMA is required to pump the molten adhesive from the storage tank to the application area and to ensure proper surface wetting when applied.
Currently used packaging adhesives comprise primarily petroleum-derived waxes such as paraffin and microcrystalline wax, and synthesis wax such as Fischer-Tropsch waxes. The lower molecular weight of paraffin wax is the primary choice when formulating low application temperature adhesives. The waxes used also generally have a crystallization temperature of less than 100° C. Due to high molecular weight of the base polymer, low molecular weight wax is necessary to reduce the viscosity of the HMA to an applicable range. Large amounts of wax are also required for set time control since wax helps control the set time. However, in certain polymers, large amounts of wax can form large segregated domains or migrate onto the bonding surface. The large difference in the molecular weight between the polymer and the wax has contributed to the wax migration.
The HMA attributes required for short set time, good bonding strength, cohesive strength and low viscosity are generally against each other. Generally selecting optimum performance is a matter of trading off one property against another, for example, reducing set time decreases adhesion at low temperature. It is challenging to obtain the desired balance among set time, bonding strength and low viscosity. Most prior art adhesives are optimized either for good adhesion, while sacrificing set time, or for a fast set time, while sacrificing adhesion at low temperatures.
The adhesion industry continues to search for adhesive compositions having a balance of properties. Accordingly, there exists a current and long felt need for adhesive compositions that overcome known deficiencies in conventional compositions and have advantageous processing properties.
It is desirable for certain hot melt adhesive compositions (e.g. packaging grades) to exhibit a good balance of fast set time and bonding strength. Good balance is achieved by selection of specific base polymers and wax blends. Adding polyethylene wax above certain levels to a polyolefin base polymer reduces set times, but at high levels can also cause a dramatic reduction in adhesion, which is sometimes referred to in terms of fiber tear (FT). This deleterious effect on adhesion may be due to surface migration of the wax. On the other hand, higher molecular weight waxes can exhibit slower surface migration relative to lower molecular weight analogs and hence better adhesion. However, the set times are longer when the adhesives are formulated with these relatively higher molecular weight waxes. Unexpectedly, the problem of improving the balance of set time versus adhesion is solved by employing a bimodal wax mixture. Blends of two or more waxes of different molecular weights, i.e., one with a relatively low weight average molecular weight (Mw) and another with a relatively high Mw, allow the formulation of adhesives of polyolefin base polymer exhibiting both improved adhesion performance and improved (reduced) set times.
U.S. discloses a hot melt adhesive composition consisting essentially of: (a) 30-70 wt % of an ethylene copolymer having about 6 to about 30 wt % of a C4 to C20 α-olefin produced in the presence of a catalyst composition comprising a metallocene and having an Mw of from about 20,000 to about 100,000; and (b) a hydrocarbon tackifier. Tse et al, U.S. Pat. No. 5,548,014, claims a hot melt adhesive composition comprising a blend of ethylene/α-olefin copolymers wherein the first copolymer has an Mw from about 20,000 to about 39,000 and the second copolymer has a Mw from about 40,000 to about 100,000. Each of the hot melt adhesives exemplified comprises a blend of copolymers, contains 45 wt % copolymer, with at least one of the copolymers having a polydispersity greater than 2.5. Furthermore, the lowest density copolymer exemplified has a specific gravity of 0.894 g/cm3, and the ARISTOWAX 165 used in the examples has a melting temperature from 68 to 74° C.
EP 0 886 656 discloses a hot melt adhesive comprising (a) from 5 to 95 weight of at least one interpolymer which is homogeneous ethylene/α-olefin interpolymer, the α-olefin being selected from the group consisting of 1-butene, 1-hexene, 4-methyl-1-pentene, 1-pentene, 1-heptene and 1-octene characterized as having a density from 0.850 to 0.885 g/cm3, and a molecular weight distribution Mw/Mn from 1.5 to 2.5; (b) from 5 to 95 wt % of at least one tackifier; (c) from 0 to about 90 wt % of at least one plasticizer; and (d) from 0 to 90 wt % of at least one wax. EP 0 886 656 also stated that waxes useful in the adhesives of its invention include paraffin waxes, microcrystalline waxes, Fischer-Tropsch, polyethylene and by-products of polyethylene wherein Mw is less than 3000.